The present invention relates in general to pressure sensors and, more particularly, to a differential pressure sensor suitable for use in a hostile environment.
Differential pressure sensors are commonly used to monitor fluid and gas pressures such as oil products, hydraulic braking, steam, radiator, air conditioning pump, and boiler pressures. In many applications, the fluid or gas is a hostile chemical such as gasoline, freons, ammonia based compounds, butanes, chlorinated solvents, alcohols, or other fuels. Typically, one side of the pressure sensor is exposed to the hostile environment. The opposite side of the sensor is exposed to a less active medium such as an air reference.
A strain gauge piezoresistive type pressure sensor is often used in hostile environments. The strain gauge type pressure sensor converts a mechanically sensed differential pressure to an electrical signal representative of the differential pressure. To protect the circuitry from the adverse environment, the sensor may be mounted within a stainless steel body that is welded shut. While the strain gauge type sensor is media and pressure compatible, it is expensive to manufacture and does not lend itself to applications where space and weight considerations are important. Additionally, it is known that the strain gauge type pressure sensor is difficult to adjust for varying temperature conditions within the application.
Another type of pressure sensor that is presently used is the ceramic capacitive pressure transducer. The ceramic capacitive pressure transducer is generally limited to low pressure applications and is less accurate than the strain gauge sensor. The ceramic capacitive sensor also requires an expensive, metal type welded housing structure.
Sensors that are designed to exhibit a high degree of accuracy often incorporate a silicon pressure sensor die. The silicon pressure sensor die resolves weight and size issues, but presents new challenges in relation to packaging for a hostile environment. Moreover, since the silicon die are inherently fragile, the packaging scheme must also include die stress relief. Attachment of the sensor die to their respective housings is accomplished by anodically bonding the sensor die to a borosilicate glass platform, or by using a flexible silicone adhesive to attach the sensor die to a package body.
In the case where borosilicate glass is used, the sensor lends itself to high pressure applications. Unfortunately, the cost of conforming the glass to a final package is often excessive. If the glass cannot be made to conform to the final package shape in a cost effective manner, and therefore cannot form a hermetic seal, the resulting device becomes media incompatible with hostile environments. The use of flexible silicone adhesives to attach pressure sensor die and provide conformal packaging is generally limited by the media compatibility of the adhesive.
Hence, a need exists for a precision differential high pressure sensor that is small in size, lightweight, low in cost, and suited for use in hostile environments.